This invention relates to a method of and apparatus for recording an image as by photography of a subject under flash illumination with optimum exposure independent of the range of the subject.
In the photographic art, exposure systems employing a diaphragm or scanning shutter having an exposure aperture area that varies with time during the exposure interval are known. Representative patents showing scanning shutters are U.S. Pat. No. 3,762,299; U.S. Pat. No. 3,972,058 and U.S. Pat. No. 4,047,190. In the above patents, a scanning shutter is described which includes a pair of counter reciprocal blades each having a primary aperture that traverses the optical axis of the camera during the exposure interval. These primary apertures are shaped so that upon overlying one another during counter movement of the blades, the exposure aperture value, defined in part by one primary aperture and in part by the other primary aperture, increases from zero to a maximum value in a preselected period of time.
In these arrangements, exposure control is exerted by a summing or integrator circuit whose resistance is constituted by a photocell located behind a photocell aperture formed by secondary apertures in the blades. Like the exposure aperture value, the photocell aperture value is also designed to change with time, such change being synchronized with the change in the exposure aperture value so as to provide proper exposure control over a wide range of conditions. The photocell aperture value which controls the scene light emitted to the photocell opens coincident with or in slightly leading arrangement to passage of first light through the exposure aperture, and when the integrator reaches a given level, a trigger circuit is fired a reverse the scan movements of the blades, which are rapidly returned to their initial position blocking passage of scene light to the photosensitive recording medium, i.e., the film.
As indicated in the above-noted patents, the diaphragm shutter system and its exposure control arrangements are designed for operation of the cameras in either an ambient or flash mode of operation, and the shape of the primary and secondary blade apertures are important to obtaining proper exposure in both modes. In these arrangements, for the flash mode of operation, the scanning blades are stopped at an aperture value preselected in accordance with the position of the camera lens and hence, are responsive to subject range. Consequently, while the aperture values are varied prior to flash firing, a preselected fixed aperture is actually provided during the flash pulse.
Scanning shutters which utilize varying apertures during the flash pulse are also known as shown by U.S. Pat. No. 3,570,381 and U.S. Pat. No. 4,020,497. In these patents, a flash fire switch is adjusted in accordance with lens focusing so that the flash will coincide with a range of aperture values; this range of aperture values being selected automatically in accordance with lens focusing and hence subject range. While these dynamic flash systems will in many instances provide satisfactory flash exposure, it should be recognized that they are dependent upon a variable switch arrangement which is subject to mechanical position errors. Additionally, in U.S. Pat. No. 4,047,191, an incandescent flash is ignited coincident with or just prior to opening of a scanning shutter with the flash illumination envelope superimposed in a leading arrangement on the aperture opening curve so as to provide increasing illumination intensity in slightly leading relation to the increasing aperture values.
One attempt to further improve flash exposure is suggested in U.S. Pat. No. 3,794,422 wherein the transmissivity of an electro-optical shutter is varied as a function of the speed of light so as to provide uniform exposure of all subjects within the flash range. In this arrangement, the flash is fired as the transmissivity of the shutter is varied from a blocking condition such that the transmissivity will be relatively low when the flash is fired but rapidly increases to a maximum. If a subject is relatively close to the camera, it will appear to be relatively brightly illuminated, however, the light reflected from that close subject will be received by the shutter when its transmissivity is relatively low. Under such condition, the shutter will relatively strongly attenuate this reflected light such that only a predetermined amount will pass. However, where the subject is relatively distant, the same amount of light will pass even though the subject will appear considerably less bright, because by the time the light is reflected from the distant subject the shutter transmissivity will have increased to a level functionally related to this dimmer light. Although the just-described arrangement is at least theoretically possible, it can be seen that the requirements for such a electro-optical shutter presents severe complications as to size, weight and expense both in terms of the shutter itself as well as the modulator required to produce the required change in transmissivity. Additionally, since the shutter functions in the order of the speed of light, the precision required for the flash device and its firing time are also unduly burdensome.
It is, therefore, an object of the present invention to provide a new and improved method of and apparatus for photographing a subject under flash illumination to achieve an optimum exposure with such transient illumination.
Another object is to provide a method of and apparatus for achieving optimum exposure under combined transient and steady state illumination.